Discharge device having cathode with micro hollow array

ABSTRACT

A discharge device for operation in a gas at a prescribed pressure includes a cathode having a plurality of micro hollows therein, and an anode spaced from the cathode. Each of the micro hollows has dimensions selected to produce a micro hollow discharge at the prescribed pressure. Preferably, each of the micro hollows has a cross-sectional dimension that is on the order of the mean free path of electrons in the gas. Electrical energy is coupled to the cathode and the anode at a voltage and current for producing micro hollow discharges in each of the micro hollows in the cathode. The discharge device may include a discharge chamber for maintaining the prescribed pressure. A dielectric layer may be disposed on the cathode when the spacing between the cathode and the anode is greater than about the mean free path of electrons in the gas. Applications of the discharge device include fluorescent lamps, excimer lamps, flat fluorescent light sources, miniature gas lasers, electron sources and ion sources.

FIELD OF THE INVENTION

[0001] This invention relates to gas discharge devices and, moreparticularly, to gas discharge devices which utilize a cathode having amicro hollow array.

BACKGROUND OF THE INVENTION

[0002] The general concept of a discharge device which utilizes a hollowcathode for increased current capability is disclosed in the prior art.A hollow cathode glow discharge utilizing a single, nearly sphericalhollow cathode is described by A.D. White in Journal of Applied Physics,Vol. 30, No. 1, May 1959, pp. 711-719. The author reported a stabledischarge and negligible deterioration from sputtering. The basicmechanisms contributing to the hollow cathode effect are described by G.Schaefer et al. in Physics and Applications of Pseudosparks, Ed. by M.A. Gundersen and G. Schaefer, Plenum Press, New York, 1990, pp. 55-76.Measurements of the temporal development of hollow cathode dischargesare described by M.T. Ngo et al. in IEEE Transactions on Plasma Science,Vol. 18, No. 3, June 1990, pp. 669-676.

[0003] A variety of hollow cathode structures for fluorescent lamps havebeen disclosed in the prior art. A directly-heated hollow cathode havingan interior coating of an emissive material is disclosed in U.S. Pat.No. 4,523,125, issued Jun. 11, 1985 to Anderson. A shielded hollowcathode for fluorescent lamps is disclosed in U.S. Pat. No. 4,461,970,issued Jul. 24, 1984 to Anderson. A hollow electrode having an interiorcoating of an emissive material that varies in thickness is disclosed inU.S. Pat. No. 2,847,605, issued Aug. 12, 1958 to Byer. A short arcfluorescent lamp having hollow cathode assemblies is disclosed in U.S.Pat. No. 4,093,893, issued Jun. 6, 1978 to Anderson. Cup shapedelectrodes containing emissive material for use in cold cathodefluorescent lamps are disclosed in U.S. Pat. No. 3,906,271, issued Sep.16, 1975 to Aptt, Jr., and U.S. Pat. No. 3,969,279, issued Jul. 13, 1976to Kern. A fluorescent lamp wherein a filament is positioned within acylindrical shield is disclosed in U.S. Pat. No. 2,549,355, issued Apr.17, 1951 to Winninghoff. Additional hollow cathode discharge devices aredisclosed in U.S. Pat. No. 1,842,215, issued Jan. 19, 1932 to Thomas;U.S. Pat. No. 3,515,932, issued Jun. 2, 1970 to King; U.S. Pat. No.4,795,942, issued Jan. 3, 1989 to Yamasaki; U.S. Pat. No. 3,390,297,issued Jun. 25, 1968 to Vollmer; and U.S. Pat. No. 3,383,541, issued May14, 1968 to Ferreira.

[0004] An electrical discharge electrode including a plurality of tubes,which are filled with an electron emissive material and embedded in ametallic matrix, is disclosed in U.S. Pat. No. 4,553,063, issued Nov.12, 1985 to Geibig et al.

[0005] A variety of. different fluorescent lamp types have beendeveloped to meet different market demands. In addition to conventionaltubular fluorescent lamps for office and commercial applications,compact fluorescent lamps have been developed as incandescent lampreplacements. Subminiature fluorescent lamps have found applications indisplays and general illumination in limited spaces.

[0006] Different fluorescent lamps may operate under very differentdischarge conditions. The small size of subminiature fluorescent lampsmay not allow for hot cathode operation, thus requiring efficient coldcathode emitters. The buffer gas pressure in subminiature fluorescentlamps is often on the order of 100 torr in order to limit electron lossto the lamp wall. By contrast, conventional fluorescent lamps typicallyutilize pressures on the order of 0.5-2.0 torr. Environmental concernshave necessitated the investigation of lamp fill materials other thanmercury. In mercury-free fluorescent lamps, radiation is often producedby excimers of inert gases, such as neon, argon and xenon. In order toform excimers, a gas pressure of approximately 100 torr is reguired. Insubminiature fluorescent lamps utilizing cold cathodes, the operatinglife may be limited by sputtering. In addition, current limitations mayrestrict light output. These trends have produced a need for improvedcathode configurations.

[0007] The hollow cathode configurations disclosed in the prior art arenot suitable for use in subminiature fluorescent lamps because of theirrelatively large sizes and because of the relatively high pressuresutilized in subminiature fluorescent lamps.

[0008] Hollow cathodes have been studied in connection with otherapplications, such as excitation sources for gas lasers, ion sources,plasma jets, electron beams and plasma switches. In each case, a cathodewith a single, relatively large opening, or hollow, has been studied atlow (subtorr) pressure.

SUMMARY OF THE INVENTION

[0009] According to the invention, a discharge device for operation in agas at a prescribed pressure comprises a cathode and an anode spacedfrom the cathode, and electrical means for coupling electrical energy tothe cathode and the anode. The cathode comprises a conductor having aplurality of micro hollows therein. Each of the micro hollows hascross-sectional dimensions selected to support a micro hollow dischargeat the prescribed pressure. Electrical energy is coupled to the cathodeand the anode at a voltage and current for producing micro hollowdischarges in each of the micro hollows in the cathode.

[0010] Each of the micro hollows preferably has a cross-sectionaldimension that is on the order of the mean free path of electrons in thegas. Under these conditions, electrons undergo oscillatory motion withinthe micro hollows and produce substantially higher currents than aplanar cathode. The micro hollow discharges operate independently ofeach other.

[0011] The prescribed pressure for operation of the discharge device ispreferably in a range of about 0.1 torr to atmospheric pressure. Thedischarge device may include a discharge chamber for maintaining theprescribed pressure. When the discharge device is operated at or nearatmospheric pressure in air, the discharge chamber may not be required.

[0012] The discharge device may include a dielectric layer between thecathode and the anode. The dielectric layer is preferably disposed of asurface of the cathode and is provided with openings aligned with themicro hollows. The dielectric layer is preferably utilized when thespacing between the cathode and the anode is greater than about the meanfree path of electrons in the gas. The dielectric layer ensures that aglow discharge between the cathode and the anode terminates in the microhollows.

[0013] According to a first application of the discharge device, afluorescent lamp comprises a sealed, light-transmissive tube containinga gas at a prescribed pressure, and first and second spaced-apartelectrodes mounted Within the tube. The first electrode comprises aconductor having a plurality of micro hollows therein. Each of the microhollows has dimensions selected to support a micro hollow discharge atthe prescribed pressure. The fluorescent lamp further includeselectrical means for coupling electrical energy to the first and secondelectrodes at a voltage and current for producing micro hollowdischarges in each of the micro hollows in the first electrode. Thefluorescent lamp preferably includes a phosphor coating on the insidesurface of the light-transmissive tube. The phosphor coating emitsradiation having a prescribed spectrum in response to radiationgenerated by the discharge between the first and second electrodes. Eachof the micro hollows preferably has a cross-sectional dimension that ison the order of the mean free path of electrons in the gas.

[0014] For AC operation of the fluorescent lamp, the second electrodepreferably comprises a conductor having a plurality of micro hollowstherein. Each of the micro hollows in the second electrode hasdimensions selected to produce a micro hollow discharge at theprescribed pressure.

[0015] The fluorescent lamp preferably includes a dielectric layer onthe surface of each electrode. Each dielectric layer has openingsaligned with the micro hollows.

[0016] In a second application of the discharge device, a radiationsource comprises a sealed discharge tube containing a gas at aprescribed pressure, first and second spaced-apart electrodes mountedwithin the discharge tube, and electrical means for coupling electricalenergy to the first and second electrodes. At least one of theelectrodes comprises a conductor having a plurality of micro hollows.Each of the micro hollows has dimensions selected to produce a microhollow discharge at the prescribed pressure. In a preferred embodiment,the radiation source is an excimer lamp wherein the gas and theprescribed pressure are selected to emit radiation in a wavelength rangeof about 80 to 200 nanometers.

[0017] In a third application of the discharge device, a laser forgenerating laser radiation at a predetermined wavelength comprises afirst mirror that is substantially reflective at the predeterminedwavelength, a second mirror that is partially reflective and partiallytransmissive at the predetermined wavelength, a chamber for enclosing agas at a prescribed pressure between the first and second mirrors, and alaser pumping device positioned between the first and second mirrors.The laser pumping device comprises a cathode having at least one microhollow therein, the micro hollow having dimensions selected to produce amicro hollow discharge at the prescribed pressure, an anode spaced fromthe cathode and electrical means for coupling electrical energy to thecathode and the anode at a voltage and current for producing the microhollow discharge in the micro hollow. The laser pumping device providesan unobstructed optical path along the optical axis between the firstand second mirrors. The cathode may include a plurality of micro hollowsand the anode may include a plurality of openings aligned with the microhollows. In this case, each of the micro hollows defines an optical axisbetween the first and second mirrors for a generation of multiple laserbeams at the predetermined wavelength. Two or more of the laser pumpingdevices may be disposed along the optical axis between the first andsecond mirrors.

[0018] In a fourth application of the discharge device, a light sourcecomprises a sealed discharge chamber containing a gas at a prescribedpressure, a cathode mounted within the discharge chamber and an anodespaced from the cathode. The cathode comprises a conductor that definesan array of micro hollows. Each of the micro hollows has across-sectional dimension selected to support a micro hollow dischargeat the prescribed pressure and has an axial dimension that issubstantially less than the cross-sectional dimension. The light sourcefurther comprises electrical means for coupling electrical energy to thecathode and the anode at a voltage and current for producing microhollow discharges in each of the micro hollows in the cathode. The lightsource is preferably configured as a thin, flat light source.

[0019] The light source may be configured as a flat fluorescent lightsource, including a phosphor coating on a light-transmissive portion ofthe discharge chamber. The phosphor coating emits radiation having aprescribed spectrum in response to radiation generated within the microhollows.

[0020] In a preferred embodiment, the cathode of the flat light sourcecomprises a wire mesh including spaced-apart conductors which define themicro hollows. Alternatively, the cathode may comprise a conductivepattern formed on a light-transmissive substrate, the conductive patterncomprising a grid of spaced-apart conductive lines.

[0021] In an additional application, the discharge device of the presentinvention can be configured as an electron source for generatingmultiple electron beams. In a further application, the discharge deviceis configured as an ion source for generating multiple ion beams.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] For a better understanding of the present invention, reference ismade to the accompanying drawings, which are incorporated herein byreference and in which:

[0023]FIG. 1 is a schematic diagram of a discharge device in accordancewith the present invention;

[0024]FIG. 2 is a graph of current as a function of voltage for thedischarge device, illustrating the high glow mode and the low glow mode;

[0025] FIG. .3 illustrates an experimental setup for evaluation of thedischarge device of the present invention;

[0026]FIG. 4 is a graph of voltage as a function of current for acathode having a single hole, and an anode-cathode separation of 2.5centimeters;

[0027]FIG. 5 is a graph of voltage as a function of current for acathode having a single hole, and an anode-cathode separation of 5centimeters;

[0028]FIG. 6 is a graph of voltage as a function of current for acathode having four holes, and an anode-cathode separation of 2.5centimeters;

[0029]FIG. 7 is a graph of voltage as a function of current for acathode having four holes, and an anode-cathode separation of 5centimeters;

[0030]FIG. 8 is a graph of voltage as a function of current for acathode having eight holes, and an anode-cathode separation of 2.5centimeters;

[0031]FIG. 9 is a graph of voltage as a function of current for acathode having eight holes, and an anode-cathode separation of 5centimeters;

[0032]FIG. 10A is a graph of voltage as a function of current for acathode having three holes and an anode-cathode separation of 0.2millimeter, for pressures in the range of 1.5 torr to 6 torr and for thelow current glow mode;

[0033]FIG. 10B is a graph of current as a function of pressure for thelow current glow mode at 320 volts, for three hole and one holedischarges;

[0034]FIG. 11 is a graph of voltage as a function of current for acathode having four holes at a pressure of two torr, showing atransition to the high current mode;

[0035]FIG. 12 is a simplified schematic diagram of a subminiaturefluorescent lamp in accordance with the present invention;

[0036]FIG. 13 is an axial view of the cathode in the subminiaturefluorescent lamp of FIG. 12;

[0037]FIG. 14 is a schematic, cross-sectional diagram of a gas laserusing the discharge device of the present invention for optical pumping;

[0038]FIG. 15 is an axial view of an array of micro hollows;

[0039]FIG. 16 is a partial cross-sectional view of a discharge devicesuitable for use as an excimer light source in accordance with thepresent invention;

[0040]FIG. 17 is a partial cross-sectional view of a flat fluorescentlight source in accordance with the present invention;

[0041]FIG. 18 is a partial illustration of the mesh cathode of FIG. 17;and

[0042]FIG. 19 is-a partial cross-sectional view of an alternateembodiment of the flat fluorescent light source.

DETAILED DESCRIPTION

[0043] A discharge device in accordance with the present invention isshown schematically in FIG. 1. The discharge device includes a cathode10 and an anode 12 mounted within a discharge chamber 14. The dischargechamber 14 is typically sealed and contains a gas at a prescribedpressure, P. The pressure P is typically in a range of about 0.1 torr toatmospheric pressure. In some cases, the discharge chamber 14 may havean opening to permit gas flow or to permit passage of a charged particlebeam, as described below. In general, the discharge chamber 14 maintainsthe pressure P between anode 10 and cathode 12 within a desired range.When the discharge device is operated at atmospheric pressure in air,the discharge chamber may be omitted. A power source 18 connected tocathode 10 and anode 12 supplies electrical energy to the dischargedevice.

[0044] The cathode 10 comprises an electrically conductive materialhaving one or more holes, referred to herein as micro hollows 20.Preferably, cathode 10 includes a plurality of micro hollows 20 forincreased current capability. The micro hollows 20 are formed in asurface 22 of cathode 10 which may be flat or curved and which facesanode 12. Each of the micro hollows 20 has a diameter, D, and extendsfrom surface 22 into cathode 10. As described below, the diameter D ofeach of the micro hollows 20 is selected to support a micro hollowdischarge at the prescribed operating pressure within discharge chamber14. The diameter D is defined as the diameter of a cross-section of themicro hollow in a plane parallel to surface 22 and perpendicular to alongitudinal axis 24 of the micro hollow 20. In some cases, the crosssection of the micro hollow may not be circular. However, for ease ofunderstanding, reference is made herein to the diameter D of the microhollow. Where the cross section is not circular, it will be understoodthat the cross-sectional dimension is selected in the manner describedbelow to support a micro hollow discharge. As shown in FIG. 1, the microhollows 20 may be closed at one end. However, the micro hollows can beopen at both ends within the scope of the present invention.

[0045] The shape of the micro hollows is not critical. The micro hollowsmay, for example, be formed by drilling, thus defining a generallycylindrical shape, at least initially. The micro hollows preferably havea circular cross section in a plane parallel to surface 22 of cathode10. Alternatively, the cross section of the micro hollow can be oval,square, rectangular or slit shaped. It has been reported that theinitial cylindrical shape of the micro hollow transforms itself into aspherical shape through sputtering and deposition. In cases where themicro hollow does not have a uniform diameter along the micro hollowaxis 24, the diameter D is defined at surface 22. The lifetime of themicro hollow cathode is expected to be long because of low cavityerosion, due to a balance of sputtering and redeposition inside themicro hollow.

[0046] The diameter D of each of the micro hollows 20 is selected tosupport a micro hollow discharge within each of the micro hollows 20.More specifically, the diameter D is selected such that the cathode fallregion extending from the inner wall of the micro hollow is on the orderof the hole radius. The cathode fall region is defined as a region ofincreased electric field near the cathode surface. The intensity anddistribution of the electric field is such that the ions acceleratedtoward the cathode gain sufficient energy to provide for emission ofsecondary electrons from the cathode, which are needed for aself-sustained glow discharge. The electrons emitted from the cathodesurface within the micro hollow are accelerated in the cathode fallregion toward the micro hollow axis 24. These electrons cross the axisand enter the cathode fall region on the opposite side of the axis,where they are reflected and accelerated across the axis again. Theoscillatory motion of the so called “pendel” electrons allows them tounload most of the energy gained in the cathode fall region throughexciting and ionizing collisions inside the micro hollow before driftingto the anode. The large ionization rate in a relatively small volumecauses a high plasma density on the discharge axis inside the microhollows 20 and consequently, a high current. A “micro hollow discharge”occurs when electrons undergo oscillatory motion within the microhollows. As used herein, the term “micro hollow” refers to a cathodehole having a cross-sectional diameter D in a plane parallel to thecathode surface. The hole diameter D times pressure P in the dischargechamber must be in a range of 0.1 torr-centimeter to 10torr-centimeters, depending on the gas type, electrode material anddesired mode of operation (high or low glow mode). In the dischargedevice of FIG. 1, the current is found to be several orders of magnitudegreater than the current for a planar cathode, and the voltage is lower.

[0047] The conditions for a micro hollow discharge as described aboveare met when the hole diameter D is on the order of the mean free pathof the electrons in the gas. The mean free path depends on the type ofgas and the gas pressure in the discharge chamber 14, and isapproximately equal to the dimension of the cathode fall region. Optimummicro hollow discharge conditions are obtained when the diameter D ofthe micro hollows is about twice the mean free path of electrons in thegas in the discharge chamber. However, it will be understood that othervalues of diameter D can be used within the scope of the invention.Preferably, the diameter D is in a range of about 0.1 to 10 times themean free path of electrons in the gas, but the diameter D is notlimited to this range.

[0048] The discharge parameters vary with the product of pressure Ptimes hole diameter D. The range of P•D for which the micro hollowdischarge is stable for rare gases was found to be on the order of 0.1to 10 torr-centimeters.

[0049] It is believed that most of the micro hollow discharge current isgenerated in a region of the micro hollow wall that extends from thesurface 22 of cathode 10 to a depth that is about three times thediameter D of the micro hollow. Thus, little additional current isobtained when the depth, L, of the micro hollow is greater than aboutthree times the diameter D. However, a micro hollow discharge occurseven when the depth L of the micro hollow is smaller than the diameterD, with a reduction in discharge current.

[0050] The number of micro hollows 20 is selected to produce a desiredtotal current at the operating voltage. It has been found that the microhollows 20 can be closely spaced without adversely affecting theindependent operation of the discharges.

[0051] Also shown in FIG. 1 is a dielectric layer 30 on surface 22 ofcathode 10. The dielectric layer 30 is required when the spacing, S,between cathode 10 and anode 12 is greater than about the mean free pathof electrons in the gas. When the spacing S exceeds this value and thedielectric layer 30 is not utilized, the glow discharge between cathode10 and anode 12 may terminate on surface 22 of cathode 10, rather thanin the micro hollows 20. Preferably,the dielectric layer 30 coverssurface 22 and surrounds micro hollows 20. The dielectric layer 30 can,for example, be a mica layer affixed to surface 22, or can be depositedon surface using known deposition techniques. When the spacing S is lessthan about the mean free path of electrons in the gas, the dielectric,layer 30 may be omitted.

[0052] The anode 12 can have any desired configuration which permits anelectric field to be established in the vicinity of cathode 10.Preferably, the anode 12 is planar and has an area that is approximatelyequal to the area of cathode 10, so that the spacing S between cathode10 and anode 12 is approximately uniform over the area of surface 22.The planar anode can optionally have holes aligned with the microhollows to provide a path for radiation generated by the discharge, agas flowing through the micro hollows, or an electron or ion beam.

[0053] In cases where the power source 18 supplies an AC voltage to thedischarge device, the anode 12 can be provided with micro hollows in thesame manner as cathode 10. To avoid confusion in the AC configuration,cathode 10 is called “electrode 10”, and anode 12 is called “electrode12”. Electrode 10 functions as a cathode during those half cycles of theAC voltage when electrode 12 is positive with respect to electrode 10,and electrode 12 functions as a cathode during those half cycles of theAC voltage when electrode 10 is positive with respect to electrode 12.By providing electrodes 10 and 12 with micro hollows as described above,micro hollow discharges are obtained on both half cycles of the ACvoltage.

[0054] The gas in the discharge chamber 14 may, for example, be an inertgas such as argon, neon or xenon. However, any desired gas can beutilized, including mixtures of gases. As noted above, the pressurewithin discharge chamber 14 is preferably in a range of about 0.1 torrto atmospheric pressure. A number of applications utilize pressures in arange of about 0.1 torr to 200 torr.

[0055] The cathode 10 can be fabricated of any desired conductivematerial. However, materials with a high rate of secondary electronemission through ion impact are preferred. Suitable materials of thistype include tungsten, barium oxide embedded in tungsten, thoriatedtungsten, molybdenum and aluminum coated with oxygen. Materials,including composite materials, characterized by a low electron workfunction are suitable as cathode materials. Other suitable materialsmeeting these requirements are known to those skilled in the art. In analternative configuration, the inside surfaces of: micro hollows 20 arecoated with materials that have high electron emissivity, and theremainder of cathode 10 is fabricated of any desired conductivematerial.

[0056] The discharge chamber 14 can have any desired size and shape.Typically, the discharge chamber is sealed to maintain pressure P in thedischarge region. The chamber 14 may be fabricated, at least in part, ofa material that transmits radiation generated by the discharge. Thus,for example, the discharge chamber 14 may be fabricated of alight-transmissive material, such as glass or quartz, or may have aradiation-transmissive window. In other embodiments, the dischargechamber 14 may be configured such that gas at pressure P flows throughthe discharge region.

[0057] The power source 18 may supply a DC voltage, a pulsed voltage oran AC voltage to the discharge device. For an AC voltage, a micro hollowdischarge occurs only on half cycles when the anode 12 is positive withrespect to the cathode 10, unless both electrodes have a micro hollowconfiguration as described above. The required voltage is typically in arange of about 300 to 600 volts. The micro hollow discharges have apositive voltage-current (V-I) characteristic over a large range ofcurrents and voltages, which permits operation of the micro hollows inparallel without ballast resistors. The micro hollow discharge has beenobserved to operate at currents up to 200 to 500 milliamps per microhollow.

[0058] The micro hollow discharges have been observed to have two glowmodes, as illustrated in FIG. 2. In a low glow mode 36 at relatively lowvoltage and current levels, the plasma column is located on the axis ofthe micro hollow and appears as a slight glow in the micro hollow. In ahigh glow mode 38, the plasma column fills almost the entire microhollow and appears as a very bright discharge in the micro hollow. Thehigh glow mode occurs at higher current and voltage levels. Thedischarge switches abruptly from the low glow mode to the high glow modeas the voltage is increased. In both modes, the discharges are stableand do not influence each other. Spectral measurements of the high glowmode indicate the presence of spectral lines from the cathode material,thereby suggesting increased sputtering of the cathode material in thehigh glow mode. Besides the gas ions, the metal ions of the sputteredelectrode material contribute to the current flow and the secondaryelectron generation at the cathode.

[0059] The high and low glow modes refer to the discharges in the microhollows 20. When the cathode 10 and the anode 12 have a spacing Sgreater than about the mean free path of electrons in the gas, a glowdischarge occurs in the region outside the micro hollows 20 betweencathode 10 and anode 12.

[0060] A set of experiments was performed to investigate micro hollowcathode discharge in an argon-mercury environment with single andmultiple cathode holes. A schematic diagram of the experimentalconfiguration is shown in FIG. 3. A test chamber was defined by a glasstube 100 having a length of 23 cm and a diameter of 4 cm. The ends ofglass tube 100 were sealed by stainless steel blocks 102 and 104. Acathode 110 and an anode 112 located within the chamber could be variedin spacing between 0.1 cm and 15 cm. Molybdenum cathodes with 1, 4 and 8holes of 0.7 mm diameter and 2.1 mm depth were used. A Cober Model 605PHigh Power Pulse Generator 116 was used to supply a 360 microsecondpulse at 30 Hz to the cathode 110. The voltage across the discharge wasmeasured using a Tektronix P-6015 100CX High Voltage Probe, and thecurrent across the load was measured using a Tektronix AM503 currentprobe.

[0061] Different gas pressures, different cold spot temperatures(mercury pressure), different electrode separations and differentnumbers of micro hollows were studied. FIG. 4 shows the voltage-current(V-I) characteristics of a cathode having a single micro hollow, with2.5 cm electrode separation,a pressure of 3 torr of a mercury-argonmixture and cold spot temperatures of 15° C. and 25° C. A constantvoltage discharge was observed at the low current level, for example,less than 240 milliamps for T=15° C. and less than 260 milliamps forT=25° C. Positive V-I characteristics were obtained at a higher currentlevel. A higher cold spot temperature promotes a lower current levelwhen the voltage is kept constant. At larger electrode separation, thatdifference disappears, and the V-I characteristics overlap. FIG. 5 showsthe V-I characteristics of a cathode having a single micro hollow, with5.0 cm electrode separation, a pressure of 3 torr and cold spottemperatures of 15° C. and 25° C. The threshold current for positive V-Icharacteristics is higher for higher cold spot temperatures, as shown inFIG. 5.

[0062]FIG. 6 shows the V-I characteristics of a cathode having fourmicro hollows, with 2.5 cm separation between electrodes, a pressure of3 torr and cold spot temperatures of 15° C. and 25° C. FIG. 7 shows theV-I characteristics of a cathode having four micro hollows, with 5.0 cmseparation between electrodes, a pressure of 3 torr and cold spottemperatures of 15° C. and 25° C. A discharge with four micro hollowsdemonstrated unstable conditions at low current levels. In the current-range below 300 milliamps at 2.5 cm electrode separation and below 350milliamps at 5.0 cm electrode separation, the four micro hollow cathodedischarge switched consecutively from the low glow mode into the highglow mode. After all of the discharges operated in the high glow mode,the V-I characteristic became positive and stable. All four microhollows were then operating in parallel with about equal lightintensity. The threshold current levels correspond to 350 volts and 375volts, respectively. The current at 25° C. was higher than at 15° C.when the voltage was maintained at a constant level.

[0063]FIG. 8 shows the V-I characteristics of a cathode having eightmicro hollows with 2.5 cm electrodes separation, a pressure of 3 torrand cold spot temperatures of 15° C. and 25° C. FIG. 9 shows the V-Icharacteristics of a cathode having eight micro hollows with 5.0 cmelectrodes separation, a pressure of 3 torr and cold spot temperaturesof 15° C. and 25° C. The cathode having eight micro hollows operated ina parallel and stable manner for currents higher than 400 milliamps,corresponding to a voltage of 375 volts, at 2.5 cm separation (FIG. 8)and for currents higher than 500 milliamps, corresponding to a voltageof 350 to 375 volts, at 5.0 cm separation (FIG. 9), after all eightmicro hollows transferred into the high glow mode. The current levelobtained with eight micro hollows is only slightly higher than with fourmicro hollows. For example, at 450 volts, four micro hollows operate at700 milliamps at 15° C. and 800 milliamps at 25° C., while eight microhollows operate at give 950 milliamps and 950 milliamps for temperaturesof 15° C. and 25° C., respectively.

[0064] Another set of experiments was performed with a cathode havingthree holes to study parallel operation of micro hollow cathodedischarge devices in a situation where the anode-cathode distance wasless than the micro hollow diameter. Cathode holes having a diameter of0.7 mm and a depth of 2.1 mm were drilled in a molybdenum disk. Amolybdenum foil 12.7 micrometers thick with four 2 mm holes was used asthe anode. The anode and cathode were separated by a 0.2 mm thick micaspacer. The voltage and current were measured as described above inconnection with FIGS. 4-9. FIG. 10A illustrates the I-V characteristicsof the three hole hollow cathode discharge with pressures between 1.5torr and 6 torr. The discharge exhibited two modes of operation, thefirst being a submilliamp unstable glow mode indicated by the pointsbelow 1.0 milliamp in FIG. 10B, and the second being a low current glowmode indicated by the points above 1.0 milliamp in FIG. 10B.- Thedischarge in the unstable glow mode was a slight glow that occupied onlythe center of the hole, and the discharge in the low current glow modeoccupied about half the hole. FIG. 10B compares the current levels at agiven voltage of the three hole discharge with a one hole discharge.Over the range of pressures shown in FIG. 10B, the ratio of three holecurrent to one hole current is about three, indicating themultiplication property of the micro hollow cathode discharge.

[0065] In another set of experiments with four holes with the samedimensions as above, the transition between the low current glow modeand the high current glow mode was observed. The high current glow modewas a very bright discharge that filled most of the hole. The dischargestarted with a low glow in each of the four holes, and as the voltageapplied to the discharge was increased, the individual holes switched tothe high glow mode. FIG. 11 shows that a low glow mode was obtained at400 volts across the discharge. The discharge current increased linearlyuntil 500 volts. Then, one of the holes transferred into the high glowmode, and the voltage decreased to 460 volts. The discharge continuedwith one hole in high glow mode until 580 volts was reached. At thispoint, a second hole transferred into the high glow mode and the voltagedecreased to 500 volts. A third hole did not transfer into the high glowmode until the voltage reached 580 volts. At that point, the voltageacross the discharge dropped to 480 volts. The fourth hole transferredto high glow mode when the discharge voltage reached 540 volts. Thedischarge voltage at this point decreased to 500 volts.

[0066] Spectra of the discharges were recorded at 3 torr argon pressure.A first spectrum was taken with all of the holes in the low glow mode,and a second was taken with three of the holes in the high glow mode.The discharges contained molybdenum lines when the high glow mode waspresent.

[0067] An application of the discharge device of the present inventionis shown in FIGS. 12 and 13. The discharge device is configured as afluorescent lamp for generation of visible light. The fluorescent lampincludes a first electrode 210 and a second electrode 212 sealed withina light-transmissive tube 214, which may be glass. The electrodes 210and 212 are spaced apart and are preferably located at or near oppositeends of light-transmissive tube 214. Electrical conductors 216 and 218extend from the exterior of light-transmissive tube 214 to electrodes210 and 212, respectively, and permit connection of electrodes 210 and212 to a source of electrical energy (not shown). The light-transmissivetube 214 defines a sealed chamber that is maintained at a desiredpressure during operation. The conductors 216 and 218 extend throughvacuum feedthroughs, as known in the art. The light-transmissive tube214 contains a fill material for supporting a low pressure dischargebetween electrodes 210 and 212. The fill material is typically an inertgas, such as neon, argon or xenon, and mercury vapor. Typically, theinside surface of light-transmissive tube 214 is coated with a phosphormaterial that emits visible light in response to ultraviolet radiationgenerated by the discharge within the tube. A variety of phosphormaterials are well known to those skilled in the art.

[0068] In the embodiment of FIGS. 12 and 13, the electrode 210 comprisesa generally disk-shaped conductor. The electrode 210 preferably has afiat surface 226 that faces electrode 212 and has sufficient thicknessfor formation of micro hollows. An array of micro hollows 230 is formedin the surface 226 of electrode 210. Each of the micro hollows 230comprises a hole having a prescribed diameter that extends from surface226 into electrode 210. The diameter of each of the micro hollows 230depends on the type of gas and the operating pressure within thedischarge device. A dielectric layer 228 is disposed on surface 226 ofelectrode 210. The dielectric layer 228 surrounds but does not covermicro hollows 230.

[0069] In the embodiment of FIGS. 12 and 13, the micro hollows 230 areclosed at one end. However, the micro hollows can extend entirelythrough electrode 210 within the scope of the present invention. Theshape of the micro hollows is not critical. The micro hollows may, forexample, be formed by drilling, thus defining a generally cylindricalshape. The electrode 210 can be fabricated of any conductive material,but is preferably fabricated of a low work function material that hashigh electron emissivity.

[0070] The diameter, D, of each of the micro hollows 230 is selected,depending on the operating pressure, P, and the gas type within thelight-transmissive tube 214, to produce a micro hollow discharge withineach of the micro hollows 230. In particular, the diameter D of each ofthe micro hollows is preferably on the order of the mean free path ofelectrons in the light-transmissive tube 214. For rare gases, thiscondition is met when the product P•D is in a range of about 0.1 to 10,where the pressure P is specified in torr and the diameter D isspecified in centimeters. The operating pressure and the type of gas areusually established by other design considerations, thus setting anallowable range of diameters for the micro hollows. Fluorescent lampstypically contain argon and mercury vapor. Conventional fluorescentlamps typically operate at pressures of 0.5 to 2.0 torr, whereassubminiature fluorescent lamps may operate at pressures of 20 to 200torr. By way of example, for a subminiature fluorescent lamp having apressure of argon and mercury in the range of 20 to 200 torr, the microhollows 230 preferably have diameters in the range of 0.5 cm to lessthan 50 micrometers. The number of micro hollows 230 is selected toproduce a desired total discharge current. Preferably, the micro hollowsare relatively uniformly distributed over surface 26, and the surface 26between micro hollows is covered by dielectric layer 228.

[0071] In the fluorescent lamp, the radiation that stimulates thephosphor coating on the light-transmissive tube 214 is generated in thepositive column between electrodes 210 and 212. The micro hollowsfunction as a source of electrons, and generation of radiation withinthe micro hollows is not important. For this reason, the micro hollowcathode is preferably operated in the low glow mode for fluorescent lampapplications.

[0072] The electrode 210 is configured to function as a cathode foremission of electrons when it is biased negatively with respect toelectrode 212. For typical fluorescent lamp applications, the electrode212 is fabricated with an array of micro hollows and a dielectric layerin the same manner as electrode 210. In this configuration, an ACvoltage is applied between conductors 216 and 218. Electrode 210functions as a cathode during those half cycles of the AC voltage whenelectrode 212 is positive with respect to electrode 210, and electrode212 functions as a cathode during those half cycles of the AC voltagewhen electrode 210 is positive with respect to electrode 212.

[0073] In another embodiment, the electrode 212 is not fabricated withan array of micro hollows and has a conventional anode configuration. Inthis embodiment, electrode 212 functions continuously as an anode, andelectrode 210 functions continuously as a cathode. A DC voltage or apulse train is applied between conductors 216 and 218 in thisconfiguration.

[0074] The electrodes 210 and 212 are preferably fabricated of amaterial with a high rate of secondary emission through ion impact.Preferred materials include tungsten, barium oxide embedded in tungsten,thoriated tungsten and molybdenum. Materials, including compositematerials, characterized by a low electron work function are suitable aselectrode materials.

[0075] A variety of different gases can be utilized in the fluorescentlamp of FIGS. 12 and 13. Preferred gases include mercury vapor mixedwith an inert gas, such as argon or krypton, an inert gas, such as neon,without mercury vapor, an excimer of an inert gas, such as Xe₂, vaporsof sulfur or selenium, and combinations thereof.

[0076] In an example, the fluorescent lamp shown in FIGS. 12 and 13 isconfigured as a subminiature fluorescent lamp. The light-transmissivetube 214 has an outside diameter of 7 mm, and the spacing betweenelectrodes-210 and 212 is about 100 mm. The tube 214 contains argon andmercury at a pressure of about 100 torr. Each of the electrodes 210 and212 has a diameter of about 5 mm and is provided with about 20 microhollows 230. The micro hollows have diameters of about 50 micrometers.The lamp is expected to operate in the low glow mode at about 300 voltsand a current of about 200 milliamps. In another example of thefluorescent lamp, the light transmissive tube 214 is 100 mm long and hasan outside diameter of 3 mm. The tube 214 contains argon and mercury ata pressure of about 50 torr. Each of the electrodes 210 and 212 has adiameter of about 1 mm and is provided with about 10 micro hollows 230.The micro hollows have diameters of about 50 micrometers. The lamp isexpected to operate in the low glow mode at about 400 volts and acurrent of about 5 milliamps per micro hollow. In general, the spacingbetween electrodes can vary between 10 cm and 100 cm, the pressure canvary between 1 and 200 torr, the micro hollow diameters can vary between10 and 1000 micrometers, and the number of micro hollows can vary from 5to 50 in order to achieve currents of 5 to 100 milliamps and voltages of20 to 500 volts. The range of selected parameters provides dischargeconditions with minimum electrode sputtering, maximum light output (10to 1000 lumens) and extended life (500 to 5000 hours). This range isdefined by subminiature fluorescent lamp conditions and applications.

[0077] The fluorescent lamp of FIGS. 12 and 13 has been described inconnection with subminiature fluorescent lamps which have relativelysmall dimensions and which operate at relatively high pressure. However,the cathode having an array of micro hollows is not limited toapplication in subminiature fluorescent lamps. The cathode having anarray of micro hollows can be used in any fluorescent lamp where theoperating pressure permits an array of suitably dimensioned microhollows to obtain desired operating characteristics. The size and numberof micro hollows is selected for a given operating pressure and currentrequirement. Furthermore, the phosphor coating on the light-transmissivetube can be omitted when the discharge within the tube produces adesired radiation spectrum. Different fill materials can be utilizedwithin the scope of the present invention. Specifically, mercury freefluorescent lamps can more easily be realized in the micro hollowcathode array system, since the expected electron energy distributionfunction is enriched with the high energy electrons and thereforepromotes excitation of higher energy levels of typical gases consideredfor mercury replacement. Ionization is also enhanced in this dischargearrangement. The cathode having an array of micro hollows can optionallybe heated to increase electron emission further.

[0078] Another application of the discharge device of the presentinvention is as an excimer lamp, which generates far ultravioletradiation, typically in the wavelength range of 80-200 nanometers. Theexcimer lamp can be used for water purification, pasteurization, wastetreatment and surface treatment of materials. The excimer lamp typicallyoperates at a relatively high pressure, on the order of 100 torr orhigher and contains a gas, such as xenon or neon, that forms dimers athigh pressures. Other suitable gases include all other noble gases andmixtures of noble gases with halogens. For operation at pressures on theorder of one atmosphere, the micro hollows have diameters on the orderof 10 to 100 micrometers. The excimer lamp can have any desiredconfiguration such as, for example, the discharge device shown inFIG. 1. Alternatively, the excimer lamp may have a configuration similarto the fluorescent lamp shown in FIG. 12 or the flat light sources shownin FIGS. 17 and 19. Generally, since the part of the discharge outsidethe micro hollows does not contribute to the excimer radiation, it canbe eliminated, thus forming a flat light source with the anode-cathodedistance shorter than the electron mean free path. All or a portion ofthe discharge chamber is fabricated of a material, such as quartz, thattransmits ultraviolet radiation at the wavelength generated within thedischarge chamber. This far ultraviolet radiation can be convertedinside the lamp into visible radiation by a specially designed phosphor.Although the efficiency of such a lamp is at present lower than theefficiency of standard fluorescent lamps, this environmentally friendlylamp fill makes it an attractive alternative.

[0079] The excimer lamp can also be implemented as a micro hollowdischarge array, as shown in FIG. 16. A conductive cathode 400 isprovided with an array of micro hollows 402, 404, 406, etc., asdescribed above in connection with FIG. 1. An anode 410 is spaced fromcathode 400 by a dielectric layer 412. A second dielectric layer 414 isformed on the opposite surface of anode 410. The anode 410 may be a thinmetal film. The anode 410 and the dielectric layers 412 and 414 may, forexample, be formed by sputter deposition on cathode 400. The anode 410and the dielectric layers 412 and 414 have openings aligned with each ofthe micro hollows 402, 404, 406, etc.

[0080] A further application of the discharge device of the presentinvention is as a miniature gas laser. As discussed above, the increasedcurrent of hollow cathode discharges compared to glow discharges betweenplanar parallel electrodes is believed to be due to the high ionizationrate of nonthermal electrons, which oscillate between opposite cathodesurfaces inside the cathode hole. The high energy electrons may be usedfor transverse pumping of miniature gas lasers, operated at gaspressures of up to one atmosphere. These miniature gas lasers, which insize are almost comparable to semiconductor lasers, may emit over a widespectral range which reaches into the ultraviolet. The hollow cathodedischarge pumped lasers are expected to be efficient. The lifetime isexpected to be long because of low cavity erosion, due to a balance ofsputtering and redeposition in the cathode hole.

[0081] The radially accelerated, nonthermal electrons in a cylindricalmicro hollow discharge unload most of their energy close to the axis ofthe cathode hole. This energy is close to the free fall energy. of theelectrons, which corresponds to the value of the applied voltage. Forsubmillimeter micro hollow discharges, the forward voltage is about 100to 500 volts. An electron energy of tens up to several hundred electronvolts is optimum for collisional ionization and excitation of atoms andmolecules. Most of the cross sections fore excitation peak at about thisvalue. When the micro hollow discharge is used for laser pumping, thehighest rate of excitation of the upper laser state is on the axis ofthe cathode hole, with a steep decay toward the wall of the microhollow. For micro hollow discharges where the initially cylindricalcathode hole may turn into a spherical one due to sputtering andredeposition of electrode material, the maximum energy deposition willoccur at the electron focal point rather than along a focal line. Inorder to avoid this nonhomogenous distribution, the cathode hole isshortened in length to a dimension that is significantly smaller thanits diameter. For a 100 micrometer diameter, a cathode hole length ofabout 25 micrometers is suitable. Transverse pumping with micro hollowcathode discharges provides a class of gas lasers which are almost ascompact as semiconductor lasers. An additional advantage of thesedevices is the low noise level of the laser intensity compared to thatof lasers pumped by conventional discharges. The noise may be reduced bytwo orders of magnitude.

[0082] The helium-neon laser is particularly appropriate for microhollow discharge pumping, because experimental results in capillarytubes with diameters of approximately 1 mm show that optimum gain isobtained when the pressure times distance product is 0.36 torr-cm, avalue close to the optimum pressure times diameter product for microhollow cathode operation. The optimum relative pressures of helium andneon depend on the discharge diameter only. For a helium-neon laserpumped with 100 micrometer diameter micro hollows, the optimum pressureis 36 torr, with 32 torr of helium and 4 torr of neon. The optimum powerof this laser is expected to be about 0.5 microwatt for a 0.5 mm long,100 micrometer diameter micro hollow cathode pumped with continuous waveenergy.

[0083] Micro hollow discharges are believed to be ideally suited as pumpsources for metal ion lasers, with metals such as cadmium, silver, gold,lead and others. Micro hollow discharges provide metal ions throughcontinuous sputtering, instead of thermal processes, to create asufficient metal vapor pressure. Lasing from ultraviolet to nearinfrared has been demonstrated with hollow cathode pumping in variousmetal ion lasers.

[0084] The micro hollow cathode discharge device of the invention mayalso be used for pumping of rare gas ion lasers. The pressure timesdistance value for rare gas ion lasers is close to the optimum pressuretimes diameter value for micro hollow discharges. A micro hollow cathodedischarge with micro hollows having diameters of 100 micrometers canpump a rare gas laser operated close to atmospheric pressure. Microhollow cathode discharges may also be used-as pump sources for nitrogenlasers and rare gas halide excimer lasers.

[0085] A cross sectional view of a single micro hollow discharge pumpedminiature gas laser is shown in FIG. 14. Micro hollow discharge elements300, 302, and 304 are stacked along an optical axis 306 of the laser.Different numbers of micro hollow discharge elements can be utilized toprovide desired laser characteristics. The discharge elements 300, 302,and 304 are positioned between a totally reflecting mirror 310 and apartially reflecting mirror 312. The partially reflecting mirror 312permits transmission of a laser beam 314 from the laser. The reflectioncharacteristics of mirrors 310 and 312 are defined at the operatingwavelength of the laser.

[0086] The discharge element 300 includes a cathode 320 and an anode 322separated by a first dielectric layer 324. A second dielectric layer 326is formed on the opposite surface of anode 322. The cathode 320 isprovided with a micro hollow 330 having a diameter that is selected,based on the type of gas and gas pressure in the discharge region, tosupport a micro hollow discharge. For operation near atmosphericpressure, the diameter of micro hollows 330 is preferably on the orderof about 10 micrometers. As noted above, the depth of the micro hollow330 is preferably less than its diameter to ensure relatively uniformpumping along optical axis 306. In a preferred embodiment, the cathode320 has a thickness on the order of 25 micrometers. The anode 322 andthe dielectric layers 324 and 326 have openings that are aligned withthe micro hollow 330 to provide an unobstructed path along axis 306. Theanode 322 and the dielectric layers 324, 326 can, for example, be formedby sputtering on cathode 320. Discharge elements 302 and 304 have thesame structure as discharge element 300. The discharge elements 300,302, and 304 are attached to each other with micro hollows 330 alignedto provide a laminated discharge structure. As noted above, more orfewer discharge elements can be utilized in the miniature gas laser ofFIG. 14.

[0087] An axial view of an array of micro hollows configured as an arrayof miniature gas lasers is shown in FIG. 15. The laser array includesarray elements 340, 342, 344, etc., each of which may be constructed asshown in FIG. 14 and described above. The laser array may have anydesired number of elements and may have a regular pattern of rows andcolumns, or may have an irregular pattern. Each discharge element of thelaser array may be constructed using conventional microlithographytechniques. The discharge elements can be bonded together to provide thelaminated structure shown in FIG. 14. The laser array generates multiplelaser beams.

[0088] In yet another application of the discharge device of the presentinvention, the micro hollow cathode discharge array is used as anelectron source or an ion source. As described above, electrons and ionsare generated within the micro hollows of the micro hollow cathode. Withreference to FIG. 16, electrons generated within micro hollows 402, 404,406, etc. are accelerated in a direction indicated by arrow 420, andions are accelerated in a direction indicated by arrow 422.

[0089] In a further application of the discharge device of the presentinvention, the micro hollow array is utilized in a thin, flat lightsource, typically a fluorescent light source. In this application, amicro hollow cathode is made of a grid of conductors, such as a wiremesh, having spacings which are preferably in the submillimeter range.The cathode is in close proximity to a planar anode. The flat lightsource can, for example, be used for backlighting of a display. Themicro hollows are formed as rings rather than cylindrical holes. Themicro hollows are implemented in accordance with the invention asdescribed above, but have small axial dimensions that are substantiallyless than their cross-sectional dimensions. The micro hollows may be,but are not required to be, open at both ends. Uniformity of thedischarge distribution in the micro hollows depends on the gas type, gaspressure, applied voltage and the mesh or grid size. While the lightsource is described as being flat, it will be understood that thecomponents can be curved in a desired contour, if desired.

[0090] Display systems which utilize liquid crystals require some formof backlighting. This is conventionally achieved by tubular fluorescentlamps with optical elements such as reflectors, collimators anddiffusers. The discharge device of the present invention is utilized byplacing an array of micro hollow discharges directly behind a phosphorcoating to achieve relatively homogeneous illumination. The micro hollowcathode may consist of a metal mesh with openings in the submillimeterrange placed between the phosphor coating and a planar metal anode.

[0091] Because of the positive voltage-current characteristics of microhollow discharges, it is possible to operate them in parallel. The microhollows do not necessarily have an extended depth in the cathodematerial but may have the form of a ring. Even the cylindrical shape ofthe micro hollows is not a precondition for micro hollow cathodedischarges. The micro hollow shape may be quadratic, rectangular orbeehive style. Thus, metal meshes with openings in the submillimeterrange can be utilized in a micro hollow cathode array. The anode can bea planar conductor separated from the cathode by a distance which iscomparable to or smaller than the cross-sectional dimensions of themicro hollows.

[0092] A partial cross-sectional view of a flat light source inaccordance with the present invention is shown in FIG. 17. A dischargechamber 500 includes a light-transmissive wall 502 and a conductive wall504. In the embodiment of FIG. 17, the light transmissive wall 502 andthe conductive wall 504 are planar, parallel, spaced-apart sheets andare closely spaced. The light-transmissive wall 502 and the conductivewall 504 are sealed around their edges to define a sealed dischargevolume. A cathode is positioned in the discharge chamber betweenlight-transmissive wall 502 and conductive wall 504. A phosphor coating506 may be applied to the inside surface of light-transmissive wall 502.A gas at a prescribed pressure is sealed within the discharge chamber500.

[0093] In the embodiment of FIG. 17, the cathode is anelectrically-conductive mesh 508. The mesh 508 comprises a grid ofspaced-apart wires or other conductive strips which define a pluralityof micro hollows. More specifically, with reference to FIG. 18, wires510, 512, 514 and 516 define a micro hollow 520. The wires 510 and 512are parallel to each other and are perpendicular to wires 514 and 516.The micro hollow 520 in the example of FIG. 18 has a squarecross-sectional shape with sides equal to the spacing between the meshwires. The axial depth of micro hollow 520 is defined by the diametersof mesh wires 510, 512, 514 and 516. The wires of the mesh 508 similarlydefine an array of micro hollows, such as micro hollows 522, 524, 526,etc. The spacing between adjacent micro hollows is determined by themesh wire diameter.

[0094] The mesh 508 is spaced from light-transmissive wall 502 by adielectric spacer 530 and is spaced from conductive wall 504 by adielectric spacer 532. It will be understood that dielectric spacers 530and 532 may be located as required to maintain a desired spacing of mesh508 with respect to light-transmissive wall 502 and conductive wall 504.The dielectric spacers 530 and 532 may, for example, be in the form ofelongated strips. In

[0095] operation, a voltage is applied between mesh 508, which functionsas the cathode of the discharge device, and conductive wall 504, whichfunctions as the anode. A micro hollow discharge is produced in each ofthe micro hollows 520, 522, 524, 526, etc. defined by the mesh 508. Theradiation generated by the micro hollow discharges stimulates emissionof visible light by phosphor coating 506. The light emitted by thephosphor coating 506 passes through light-transmissive wall 502 andappears as a generally uniform planar light source.

[0096] In the light source of FIG. 17, the fill gas may be a noble gaswith mercury vapor, with dominant emission in the ultraviolet. Othersuitable gases include inert gases, such as xenon, krypton and argon, ortheir excimers which would emit ultraviolet radiation, visible radiationor a combination of visible and ultraviolet radiation. The micro hollowcathode discharge enhances the high energy tail in the electron energydistribution function, allowing for more efficient excitation of excimerstates than conventional discharges. Molecular gases, such as nitrogen,oxygen or air, and sulfur or selenium vapors, and their mixtures withinert gases, may be used in the flat light source. The gas pressuredepends on the diameter of the cathode holes. For a mesh with 200micrometer openings and 50 micrometer wire diameter, the pressure ispreferably in a range of about 10 to 500 torr. The applied voltage is onthe order of 400 volts DC or pulsed. Preferably, the mesh spacing is ina range of 10-500 micrometers, which depends on the gas and gaspressure. In cases where it is not necessary to change the wavelength ofthe radiation generated in the micro hollows, the phosphor coating 506may be omitted.

[0097] Experiments were performed to study the gas discharge between aplanar electrode and a mesh electrode for utilization as a flat lightsource. The experimental setup included a vacuum chamber which includeda planar anode made of tungsten impregnated with barium, and a nickelmesh with quadratic openings of 0.206 mm width separated by 0.044 mmwide metal bars, or strips, of 0.0014 mm thickness. The spacing betweenthe electrodes was on the order of 0.15 mm, determined by a mica spacerhaving an opening of about 2.5 mm. The gas was air at a pressure of 37.5torr. A voltage pulse with a droop of about 10% over the entire durationof 0.4 milliseconds was applied to the electrodes, and the currentthrough the discharge was monitored with a current viewing resistor.Simultaneously, the discharge was observed by a CCD camera connected toa magnifying system.

[0098] The results were as follows. At an applied voltage of 384 voltsand with the mesh biased negatively, the current at the beginning of thepulse was measured at 33 milliamps. The current decayed to about halfthis value over the duration of the voltage pulse, indicating anonlinear dependence of the current on the voltage. Discharges developedin the mesh. openings. Two types of discharges were observed a dimdischarge centered in the mesh openings in a majority of the holes, anda bright, centered discharge in a small number of holes. With anincreasing number of pulses, the dim discharges became brighter, and theinitially bright discharges lost intensity. The current did not changesignificantly during the transition phase from inhomogeneous to morehomogeneous light distribution. Continuous operation in this mode leadsto substantial erosion of the mesh. In another experiment after morethan 500,000 pules, the 0.0014 mm bar was completely eroded at theposition of the brightest discharge. These results indicate that theflat light source needs to be operated in a low glow mode to avoiderosion. This also guarantees long lifetime and preferable operation forlighting applications. Experiments with reverse polarity (the planarelectrode 504 functioning as the cathode) showed a homogeneous glow at alower current of 16 milliamps and reduced intensity at the same voltageand pressure indicated above.

[0099] An alternate embodiment of the flat light source is shown in FIG.19. Like elements in FIGS. 17 and 19 have the same reference numerals.In the embodiment of FIG. 19, a cathode 550 is formed as a conductivepattern on a transparent substrate 552. The cathode 550 includes a gridof spaced-apart conductive lines 556, 558, 560, etc. which define microhollows 564, 566, etc. The conductive pattern of cathode 550 can haveany desired configuration for defining a plurality of micro hollows. Theconductive pattern may formed using conventional microlithographytechniques. In the embodiment of FIG. 19, the substrate 552 functions asa support for the cathode 550. In other respects, the discharge deviceof FIG. 19 is similar to the discharge device shown in FIG. 17 anddescribed above.

[0100] The flat light sources of FIGS. 17-19 may have a thickness on theorder of one millimeter. As noted above, the light sources shown inFIGS. 17-19 may be flat or may have a desired curvature.

[0101] Generally, the devices of the present invention can be operatedin the low glow mode and the high glow mode as described above. However,only the low glow mode promises long lifetimes and operation determinedby the fill gas. In the high glow mode, the lifetime is limited, and theelectrode vapor determines the characteristic of the discharge. This maybe desirable when metal vapor radiation is required.

[0102] While there have been shown and described what are at presentconsidered the preferred embodiments of the present invention, it willbe obvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

What is claimed is:
 1. A discharge device comprising: a dischargechamber containing a gas at a prescribed pressure; a cathode mountedwithin said discharge chamber, said cathode comprising a conductorhaving a plurality of micro hollows therein, each of said micro hollowshaving dimensions selected-to support a micro hollow discharge at saidprescribed pressure; an anode mounted within said discharge chamber andspaced from said cathode; and electrical means for coupling electricalenergy to said cathode and said anode at a voltage and current forproducing micro hollow discharges in each of the micro hollows in saidcathode.
 2. A discharge device as defined in claim 1 wherein each ofsaid micro hollows has a cross-sectional dimension that is on the orderof the mean free path of electrons in said gas.
 3. A discharge device asdefined in claim 1 wherein each of said micro hollows has across-sectional dimension that is in a range of about 0.1 to 10 timesthe mean free path of electrons in said gas.
 4. A discharge device asdefined in claim 1 further including a dielectric layer between saidcathode and said anode, said dielectric layer having openings alignedwith said micro hollows.
 5. A discharge device as defined in claim 4wherein said dielectric layer is positioned on a surface of saidcathode.
 6. A discharge device as defined in claim 1 wherein saidprescribed pressure is in a range of about 0.1 torr to atmosphericpressure.
 7. A discharge device as defined in claim 1 wherein saiddischarge chamber is sealed.
 8. A discharge device comprising: adischarge chamber containing a gas at a prescribed pressure; a cathodemounted within said discharge chamber, said cathode comprising aconductor having at least one micro hollow therein, said micro hollowhaving dimensions selected to support a micro hollow discharge at saidprescribed pressure; an anode mounted within said discharge chamber andspaced from said cathode; a dielectric layer between said, cathode andsaid anode, said dielectric layer having an opening aligned with said atleast one micro hollow; and electrical means for coupling electricalenergy to said cathode and said anode at a voltage and current forproducing said micro hollow discharge in said at least one micro hollow.9. A. discharge device as defined in claim 8 wherein said at least onemicro hollow has a cross-sectional. dimension that is on the order ofthe mean free path of electrons in said gas.
 10. A discharge device asdefined in claim 8 wherein said at least one micro hollow has across-sectional dimension that is in a range of about 0.1 to 10 timesthe mean free path of electrons in said gas.
 11. A discharge device asdefined in claim 8 wherein said cathode includes a plurality of microhollows.
 12. A discharge as defined in claim 11 wherein said dielectriclayer is located on a surface of said cathode and includes a pluralityof openings aligned with said micro hollows.
 13. A discharge device asdefined in claim 8 wherein said discharge chamber is sealed.
 14. Adischarge device as defined in claim 8 wherein said cathode includes asurface facing said anode and wherein said at least one micro hollow isformed in said surface.
 15. A discharge device as defined in claim 8wherein said at least one micro hollow has a generally cylindricalinitial shape and is open at one end facing said anode.
 16. A dischargedevice as defined in claim 8 wherein. said at least one micro hollow hasa generally cylindrical initial shape and is open at both ends.
 17. Adischarge device for operation in a gas at a prescribed pressure,comprising: a cathode comprising a conductor having a plurality of microhollows therein, each of said micro hollows having dimensions selectedto produce a micro hollow discharge at said prescribed pressure; ananode spaced from said cathode; and electrical means for couplingelectrical energy to said cathode and said anode at a voltage andcurrent for producing micro hollow discharges in each of the microhollows in said cathode.
 18. A discharge device as defined in claim 17wherein each of said micro hollows has a cross-section with dimensionsthat are on the order of the mean free path of electrons in said gas.19. A discharge device as defined in claim 17 further including adielectric layer between said cathode and said anode, said dielectriclayer having openings aligned with said micro hollows.
 20. A dischargedevice as defined in claim 19 wherein said dielectric layer is formed onsaid cathode and wherein said anode comprises a conductive layer formedon said dielectric layer.
 21. A discharge device as defined in claim 17wherein said prescribed pressure is in a range of about 0.1 torr toatmospheric pressure.
 22. A discharge device as defined in claim 17wherein each of said micro hollows has a cross section with dimensionsin a range of about 1 centimeter to 10 micrometers.
 23. A cathode foruse in a discharge device that operates in a gas at a prescribedpressure, comprising a conductive element having a surface and at leastone micro hollow formed in said surface, said at least one micro hollowhaving a diameter that is on the order of the mean free path ofelectrons in said gas, said cathode further comprising a dielectriclayer on said surface, said dielectric layer having an opening alignedwith said at least one micro hollow.
 24. A cathode for use in adischarge device that operates in a gas at a prescribed pressure,comprising a conductive element having a surface and a plurality ofmicro hollows formed in said surface, each of said micro hollows havinga cross-sectional dimension that is on the order of the mean free pathof electrons in said gas.
 25. A cathode as defined in claim 24 furthercomprising a dielectric layer on said surface of said conductiveelement, said dielectric layer having opening aligned with said microhollows.
 26. A cathode as defined in claim 24 wherein each of said microhollows has a depth of at least three times the cross-sectionaldimension of each of said micro hollows.
 27. A cathode as defined inclaim 24 wherein each of said micro hollows has a generally cylindricalinitial shape and is open at one end.
 28. A cathode as defined in claim24 wherein each of said micro hollows has a generally cylindricalinitial shape and is open at both ends.
 29. A light source comprising: asealed, light-transmissive tube containing a gas at a prescribedpressure, P; a first electrode mounted within said tube, said firstelectrode comprising a conductor having a plurality of micro hollowstherein, each of said micro hollows having dimensions selected tosupport a micro hollow discharge at said prescribed pressure; a secondelectrode mounted within said tube and spaced from said first electrode;and electrical means for coupling electrical energy to said first andsecond electrodes at a voltage and current for producing micro hollowdischarges in each of the micro hollows in said first electrodes.
 30. Alight source as defined in claim 29 wherein each of said micro hollowshas a diameter, D, such that P•D is in a range of about 0.1 to 10torr-centimeters.
 31. A light source as defined in claim 29 wherein eachof said micro hollows has a cross-sectional dimension that is on theorder of the mean free path of electrons in said gas.
 32. A light sourceas defined in claim 29 wherein each of said micro hollows comprises avolume enclosed by the conductor of said first electrode except for anopening facing said second electrode.
 33. A light source as defined inclaim 29 wherein said prescribed pressure is in a range of about 0.1 to200 torr.
 34. A light source as defined in claim 33 wherein each of saidmicro hollows has a diameter in a range of about 10 micrometers to 1centimeter.
 35. A light source as defined in claim 29 wherein theconductor of said first electrode is a material selected from a groupconsisting of tungsten, thoriated tungsten and molybdenum.
 36. A lightsource as defined in claim 29 wherein, the conductor of said firstelectrode is a composite material characterized by a low electron workfunction.
 37. A light source as defined in claim 29 wherein said firstelectrode includes a surface facing said second electrode and whereinsaid micro hollows are formed in said surface.
 38. A light source asdefined in claim 29 wherein said gas comprises argon and mercury vapor.39. A light source as defined in claim 29 wherein said first electrodefurther includes a dielectric layer on said conductor, said dielectriclayer having openings aligned with said micro hollows.
 40. A lightsource as defined in claim 38 further including a phosphor coating on aninside surface of said light-transmissive tube, said phosphor coatingemitting radiation having a prescribed spectrum in response to radiationgenerated within said tube.
 41. A light source as defined in claim 29wherein said second electrode comprises a conductor having a pluralityof micro hollows therein, each of said micro hollows having dimensionsselected to produce a micro hollow discharge at said prescribedpressure.
 42. A light source as defined in claim 41 wherein saidelectrical means comprises means for coupling AC electrical energy tosaid first and second electrodes.
 43. A light source as defined in claim29 wherein said electrical means comprises means for coupling DCelectrical energy to said first and second electrodes.
 44. A lightsource as defined in claim 29 wherein said electrical means comprisesmeans for coupling pulsed electrical energy to said first and secondelectrodes.
 45. A light source as defined in claim 29 configured as asubminiature fluorescent lamp wherein said prescribed pressure is in arange of about 1 to 200 torr and each of said micro hollows has adiameter less than 1 millimeter.
 46. A light source as defined in claim29 wherein said gas is selected from the group consisting of an inertgas, mercury vapor mixed with an inert gas, an excimer of an inert gas,sulfur vapor, selenium vapor and combinations thereof.
 47. A fluorescentlamp comprising: a sealed, light-transmissive tube containing a gas at aprescribed pressure, P; first and second spaced-apart electrodes mountedwithin said light-transmissive tube, each of said electrodes comprisinga conductor having a plurality of micro hollows therein, each of saidmicro hollows having dimensions selected to produce a micro hollowdischarge at said prescribed pressure; a dielectric layer on a surfaceof each of said first and second electrodes, each of said dielectriclayers having openings aligned with said micro hollows; a phosphorcoating on an inside surface of said light-transmissive tube, saidphosphor coating emitting radiation having a prescribed spectrum inresponse to radiation generated within said tube; and electricalconductors for coupling electrical energy to said first and secondelectrodes at a voltage and current for producing micro hollowdischarges in each of the micro hollows in said first and secondelectrodes.
 48. A fluorescent lamp as defined in claim 47 wherein eachof said micro hollows has a cross-sectional dimension that is on theorder of the mean free path of electrons in said gas.
 49. A fluorescentlamp as defined in claim 47 wherein each of said micro hollows has adiameter, D, such that P•D is in a range of about 0.1 to 10 torr-cm. 50.A fluorescent lamp as-defined in claim 47 wherein said prescribedpressure is in a range of about 0.1 to 200 torr.
 51. A radiation sourcecomprising: a sealed discharge chamber containing a gas at a prescribedpressure; first and second spaced-apart electrodes mounted within saiddischarge chamber, at least one of said electrodes comprising aconductor having a plurality of micro hollows therein, each of saidmicro hollows having dimensions selected to produce a micro hollowdischarge at said prescribed pressure; and electrical means for couplingelectrical energy to said first and second electrodes at a voltage andcurrent for producing micro hollow discharges in each of said microhollows and for producing a radiation-emitting discharge between saidfirst and second electrodes, said discharge chamber including at least aportion for transmitting radiation.
 52. A radiation source as defined inclaim 51 wherein said gas and said prescribed pressure are selected toemit excimer radiation in a wavelength range of about 80-200 nanometers.53. A radiation source as defined in claim 51 wherein each of said microhollows has a cross-sectional dimension that is on the order of the meanfree path of electrons in said gas.
 54. A laser for generating laserradiation at a predetermined wavelength comprising: a first mirror thatis substantially reflective at said predetermined wavelength; a secondmirror that is partially reflective and partially transmissive at saidpredetermined wavelength, said first and second mirrors being spacedapart and having a parallel orientation; a chamber for enclosing a gasat a prescribed pressure between said first and second mirrors; and alaser pumping device positioned between said first and second mirrors,said laser pumping device comprising a cathode having at least one microhollow therein, said at least one micro hollow having dimensionsselected to produce a micro hollow discharge at said prescribedpressure; an anode spaced from said cathode; and electrical means forcoupling electrical energy to said cathode and said anode at a voltageand current for producing said micro hollow discharge in said at leastone micro. hollow, said anode and, said at least one micro hollowproviding an unobstructed optical path along an optical axis betweensaid first and second mirrors.
 55. A laser as defined in claim 54wherein said cathode includes a plurality of micro hollows and saidanode includes a plurality of openings aligned with said micro hollows,each of said micro hollows defining an optical axis between said firstand second mirrors for generation of parallel beams of laser radiationat said predetermined wavelength.
 56. A laser as defined in claim 54including two or more of said laser pumping devices disposed along saidoptical axis between said first and second mirrors.
 57. A device forgenerating a plurality of electron beamsi comprising: a dischargechamber containing a gas at a prescribed pressure, a cathode mountedwithin said discharge chamber, said cathode comprising a conductorhaving a plurality of micro hollows therein, each of said micro hollowshaving dimensions selected to produce a micro hollow discharge at saidprescribed pressure; an anode mounted within said discharge chamber andspaced from said cathode, said anode having openings aligned with saidmicro hollows; and electrical means for coupling electrical energy tosaid cathode and said anode at a voltage and current for producing microhollow discharges in each of the micro hollows in said cathode, wherebysaid micro hollows generate multiple electron beams.
 58. A device forgenerating a plurality of ion beams, comprising: a discharge chambercontaining an ionizable gas at a prescribed pressure; a cathode mountedwithin said discharge chamber, said cathode comprising a conductorhaving a plurality of micro hollows therein, each of said micro hollowshaving dimensions selected to produce a micro hollow discharge at saidprescribed pressure; an anode mounted within said discharge chamber andspaced from said cathode; and electrical means for coupling electricalenergy to said cathode and said anode at a voltage and current forproducing micro hollow discharges in each of the micro hollows in saidcathode, whereby said micro hollows generate multiple ion beams.
 59. Alight source comprising: a sealed discharge chamber containing a gas ata prescribed pressure; a cathode mounted within said discharge chamber,said cathode comprising a conductor that defines an array of microhollows, each of said micro hollows having a cross-sectional dimensionselected to support a micro hollow discharge at said prescribed pressureand having an axial dimension that is substantially less than saidcross-sectional dimension; an anode spaced from said cathode; andelectrical means for coupling electrical energy to said cathode and saidanode at a voltage and current for producing micro hollow discharges ineach of the micro hollows in said cathode.
 60. A light source as definedin claim 59 wherein said cathode comprises a wire mesh includingspaced-apart conductors which define said micro hollows.
 61. A lightsource as defined in claim 59 wherein said discharge chamber includes alight-transmissive portion for transmission of light generated in saidmicro hollows.
 62. A light source as defined in claim 61 wherein saidcathode is located between the light-transmissive portion of saiddischarge chamber and said anode.
 63. A light source as defined in claim62 further including a phosphor coating on the light-transmissiveportion of said discharge chamber, said phosphor coating emittingradiation having a prescribed spectrum in response to radiationgenerated within said micro hollows.
 64. A light source as defined inclaim 60 wherein said wire mesh comprises a grid of spaced-apartconductors having spacings of about 200 micrometers and diameters ofabout 40 micrometers.
 65. A light source as defined in claim 59 whereinsaid cathode comprises a conductive pattern formed on alight-transmissive substrate, said conductive pattern comprising a gridof spaced-apart conductive lines.
 66. A light source as defined in claim59 wherein said discharge chamber has a thin, flat configuration,thereby defining a flat light source.
 67. A light source as defined inclaim 59 wherein said prescribed pressure is in a range of about 10 to500 torr.
 68. A light source comprising: a sealed discharge chambercontaining a gas at a prescribed pressure, said discharge chamberincluding a flat, light-transmissive wall and a metal wall spaced fromand parallel to said light-transmissive wall, said metal wall comprisingan anode; a cathode mounted within said discharge chamber between saidlight-transmissive wall and said metal wall, said cathode comprising aconductor that defines an array of micro hollows, each of said microhollows having a cross-sectional dimension selected to support a microhollow discharge at said prescribed pressure and having an axialdimension that is substantially less than said cross-sectionaldimension; and electrical means for coupling electrical energy to saidcathode and said anode at a voltage and current for producing microhollow discharges in each of the micro hollows in said cathode.
 69. Alight source as defined in claim 68 wherein said cathode comprises awire mesh including a grid of spaced-apart electrical conductors whichdefine said micro hollows.